An acid (from Latin acidus, meaning sour or tart) is a chemical compound generally defined by its reactions with complementary chemicals, designated bases (see Acid-base reaction theories). An acid tends to give a proton and can be represented by the generic formula AH. In water, there is the following reaction:

Some of the stronger acids include the hydrohalic acids - HCl, HBr, and HI - and the oxyacids, which tend to contain central atoms in high oxidation states surrounded by oxygen - including HNO3, H2SO4, HClO4.

The Swedish chemist Svante Arrhenius defined an acid to be a substance that gave up hydrogen ions (H+) when dissolved in water, while bases are substances that give up hydroxide ions (OH-). Notice that this definition limits acids and bases to substances that can dissolve in water. Later on, Bronsted[?] and Lowry[?] defined an acid to be a proton donor and a base to be a proton acceptor. In this definition, even substances that are insoluble in water can be acids and bases. The most general definition of acids and bases is the Lewis definition. A Lewis acid is an electron acceptor, while a Lewis base[?] is an electron donor. Acid/base systems are different from redox reactions in that there is no change in oxidation state.

"Acid" is also the sound that a Roland TB-303 music synthesizer makes, and is where the terms Acid Jazz[?] and Acid House originate. A very important term in electronic music, often confused with the LSD meaning.